Electronic devices such as servers and personal computers are making remarkable progress to achieve higher throughput and performances in recent years.
Further, a recent trend in semiconductor devices such as semiconductor chips and semiconductor packages which are the central heart of a computer is to enhance their performances by achieving higher circuit densities and growing in size for larger capacities.
Further, a semiconductor device can be mounted on a wiring board by the use of a method for flip chip bonding. According to this mounting method, an electrode provided on a wiring board is soldered and connected with an electrode provided on a semiconductor device.
See, e.g., Japanese Laid-open Patent Publications Nos. 2004-342959, 08-236898 and 10-12990.
Incidentally, according to the above mounting method, put an electrode provided to a semiconductor device on an electrode provided to a wiring board with solder provided between the electrodes, and then melt the solder so as to mount the semiconductor device on the wiring board by soldering and connecting the electrode of the semiconductor device with the electrode of the wiring board.
As illustrated in FIG. 14A, e.g., put a solder bump 105 provided on an electrode 104 of a semiconductor device 103 on a solder paste 102 applied to an electrode 101 of a wiring board 100. Then, melt the solder paste 102 and the solder bump 105 so as to mount the semiconductor device 103 on the wiring board 100 by soldering and connecting the electrode 104 of the semiconductor device 103 with the electrode 101 of the wiring board 100, as illustrated in FIG. 14B. In this case, a mounting structure 106 in which the semiconductor device 103 is mounted on the wiring board 100 is formed by the electrode 104 of the semiconductor device 103 connected with the electrode 101 of the wiring board 100 by a solder piece 107.
In this case, the solder solidifies while surface tension is balancing self weight of the semiconductor device 103 in time of solder melting. Thus, the solder piece 107 which connects the electrode 104 of the semiconductor device 103 with the electrode 101 of the wiring board 100 is shaped like a squashed ball or a drum, i.e., a drum expanded in the middle in a vertical direction.
If the semiconductor device 103 generates heat while working and is thermally expanded, a thermal expansion difference occurs between the semiconductor device 103 and the wiring board 100. The thermal expansion difference causes stress to be applied to the solder connection connecting the electrode 104 of the semiconductor device 103 with the electrode 101 of the wiring board 100.
The thermal expansion difference causes large stress to be applied to a solder connection placed on an outer portion, in particular.
Further, if the solder piece 107 is shaped like a drum as described above, the stress tends to be concentrated on portions where the solder piece 107 comes into contact with the electrodes 101 and 104 as illustrated in FIG. 14C. Incidentally, a portion indicated with a symbol XIVC in FIG. 14B is expanded and illustrated in FIG. 14C.
Further, if the semiconductor device 103 is turned on and off, the stress is repeatedly applied to the solder connection between the electrode 104 of the semiconductor device 103 and the electrode 101 of the wiring board 100.
Thus, it is not preferable for the solder connection on the outer portion to which large stress caused by the thermal expansion difference is applied to have a portion that the stress is concentrated on.